linux-kernel - Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON

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Message-ID: <CAKv+Gu-cOV6uo6MpLxr9uQSTT-ku_wn9+D7CC4+3maJtvcaGoA@mail.gmail.com>
Date:   Wed, 18 Jul 2018 23:50:39 +0900
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Kees Cook <keescook@...omium.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Alasdair Kergon <agk@...hat.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Lars Persson <larper@...s.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Rabin Vincent <rabinv@...s.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, qat-linux@...el.com,
        dm-devel@...hat.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK

On 18 July 2018 at 05:59, Arnd Bergmann <arnd@...db.de> wrote:
> On Sun, Jul 15, 2018 at 6:28 AM, Kees Cook <keescook@...omium.org> wrote:
>>
>> After my ahash to shash conversions, only ccm is left as an ahash
>> user, since it actually uses sg. But with the hard-coded value reduced
>> to 376, this doesn't trip the frame warnings any more. :)
>>
>> I'll send an updated series soon.
>
> Maybe we should get rid of that one as well then and remove
> AHASH_REQUEST_ON_STACK()?
>
> I see that Ard (now on Cc) added this usage only recently. Looking
> at the code some more, I also find that the descsize is probably
> much smaller than 376 for all possible cases   of "cbcmac(*)",
> either alg->cra_blocksize plus a few bytes or sizeof(mac_desc_ctx)
> (i.e. 20) for arch/arm64/crypto/aes-glue.c.
>
> Walking the sglist here means open-coding a shash_ahash_update()
> implementation in crypto_ccm_auth(), that that doesn't seem to
> add much complexity over what it already has to do to chain
> the sglist today.
>

It would be better to add a variably sized ahash request member to
struct crypto_ccm_req_priv_ctx, the only problem is that the last
member of that struct (skreq) is variably sized already, so it would
involve having a struct ahash_request pointer pointing into the same
struct, after the skreq member.